What is gn?

GN is a build system generator used primarily by the Chromium project. It's designed to be faster and simpler to use than its predecessor, GYP. Here's a breakdown:

• Purpose: GN takes human-readable build descriptions and generates native build files (e.g., Makefiles, Ninja files, Xcode projects, Visual Studio solutions) that a build tool like Ninja can use to perform the actual compilation and linking.
• Key Features:
  • Simplicity: GN strives for a relatively simple syntax compared to other build systems. It emphasizes readability and maintainability of build files.
  • Speed: GN is designed to be significantly faster than GYP in terms of build file generation time. This is important for large projects like Chromium.
  • Declarative: GN files declare what you want to build and the dependencies between components. It's less about imperative scripting compared to other build systems.
  • Templates: GN supports the use of templates to reduce code duplication and simplify the build description process. You can define reusable blocks of code for common build patterns.
  • Functions: GN uses functions for many operations. Some built-in functions are for creating targets, setting variables, or processing lists.
  • Scope: Variables and settings defined in a BUILD.gn file have a scope, meaning they're only applicable within a particular context. This helps avoid naming conflicts and makes the build process more predictable.
  • Integration with Meta-Build Systems: GN is often used as a backend for more complex build systems. For example, it can be used with tools that generate build configuration from higher-level specifications.
• Files: The main file in GN is called BUILD.gn, where you define your targets. A BUILDCONFIG.gn file defines global build settings and toolchains.
• Example: A simple BUILD.gn file might look something like this:

executable("my_program") {
  sources = [ "my_program.cc" ]
  deps = [ "//mylibrary" ]
}

This defines an executable named "my_program" that depends on the target "//mylibrary".

• Relationship to Ninja: GN generates Ninja build files. Ninja is then responsible for executing the actual build process. Ninja is known for its speed and efficiency in performing incremental builds.

Important Concepts:

• Targets: Define the buildable units in your project (e.g., executables, libraries, tests).
• Sources: The source files that are used to build a target.
• Dependencies: Relationships between targets; one target might require another target to be built first.
• Variables: Used to store configuration options and settings within the build files.
• Toolchains: Define the compiler, linker, and other tools used to build the code.
• Templates: Reusable blocks of build configuration.
• Build%20Arguments: Values that can be passed to the build system to control the build configuration.